At present, soft drinks such as juices, or alcoholic beverages such as beer, are sold in a state filled in containers such as bottles or pet bottles. In such a case, in order to differentiate the products from other products to improve visibility of the products, it is common to apply printed heat-shrinkable labels on the outside of the containers. As the base material for such heat-shrinkable labels, polyvinyl chloride, polyester or polystyrene may, for example, be usually employed.
A polyvinyl chloride (hereinafter referred to as “PVC”) heat-shrinkable film has good shrink finishing quality and little natural shrinkage (i.e. the natural shrinkage being small) and thus has, heretofore, been widely used as a heat-shrinkable label. However, at the time of incineration after the use, it is likely to cause generation of a hazardous gas such as hydrogen chloride, dioxin, etc. Accordingly, from the viewpoint of environmental preservation in recent years, research and development have been carried out for a heat-shrinkable film using a material to be substituted for such PVC. On the other hand, in the application to labels for pet bottles which are expected to be more demanded, a heat-shrinkable film having little natural shrinkage is required whereby a high level of shrink finishing appearance can be obtained in a relatively short time at a relatively low temperature. The reason may, for example, be the need for a low temperature in the labeling step of a shrink film to be put on a recent pet bottle. Namely, at present, it is most common to employ a method of labeling which comprises shrinking a heat-shrinkable film by means of a vapor shrinker. However, in order to avoid deterioration of the quality due to aseptic filling or temperature rise of the content, it is desirable to carry out the shrink step at a temperature as low as possible. For such a reason, in the current shrink film industry, research and development are being carried out for a heat-shrinkable film which starts shrinkage at a temperature as low as possible in a vapor shrinker at the time of labeling and whereby after the film was passed through the vapor shrinker, an excellent shrink finishing property can be obtained.
In such an application, a polyester heat-shrinkable film is mainly used which is rigid at room temperature and has low temperature shrinkage and very little natural shrinkage. However, as compared with the PVC heat-shrinkable film, the polyester heat-shrinkable film has had a problem that at the time of the heat shrinkage, shrink marks or wrinkles are likely to form.
On the other hand, in order to overcome the problems of the above-mentioned PVC and polyester heat-shrinkable films, a polystyrene heat-shrinkable film containing a styrene-butadiene block copolymer (SBS) as the main material has been proposed and used. Such a polystyrene heat-shrinkable film has a merit in that the shrink finishing quality is good as compared with the PVC and polyester heat-shrinkable films, but, on the other hand, has had problems such that the elasticity is weak, and it is poor in natural shrinkage. Therefore, it has been attempted to develop a styrene heat-shrinkable film which is capable of solving these problems.
As a means to solve the above problems, for example, as a film for packaging material, a laminate film has been reported wherein an outer layer made of a polyester resin is laminated via an adhesive layer on an intermediate layer made of a polystyrene resin (Patent Document 1). However, such a laminate film has had a problem that during the shrinkage of the film, the adhesive layer cannot follow other layers whereby an appearance failure represented by delamination will result.
Further, a shrink label has been reported which is provided with a base film wherein front and rear layers made of a polyester resin containing 1,4-cyclohexanedimethanol as a diol component, are laminated on both sides of an intermediate layer made of a polystyrene resin (Patent Documents 2 and 3). However, such a shrink label has had problems such that the interlaminar adhesion is inadequate, and delamination is likely to result during secondary processing or during printing.
Further, as a technique having the interlaminar adhesion improved, a film has been reported wherein a block copolymer of a vinyl aromatic hydrocarbon with a conjugated diene is used for an inner layer, a copolymer polyester is used for both outer layers, and an ethylene-vinylacetate copolymer, an ethylene-unsaturated carboxylic acid copolymer, or the like, is used for an adhesive layer (Patent Document 4). However, such a film has had a problem that the compatibility between the vinyl aromatic hydrocarbon and the conjugated diene derivative of the inner layer and the ethylene-vinylacetate copolymer of the adhesive layer is poor, whereby the transparency of the entire film tends to be low when a recycled resin formed from a trimming loss of a film edge is incorporated (hereinafter referred to as “incorporated as recycled”).
Patent Document 1: JP-A-61-41543
Patent Document 2: JP-A-2002-351332
Patent Document 3: JP-A-7-137212
Patent Document 4: JP-B-5-33895